Screw pumps are potentially attractive since they can be manufactured with few working components and they have an ability to pump from a high vacuum environment at the inlet down to atmospheric pressure at the outlet. Screw pumps usually comprise two spaced parallel shafts each carrying externally threaded rotors, the shafts being mounted in a pump body such that the threads of the rotors intermesh. Close tolerances between the rotor threads at the points of intermeshing and with the internal surface of the pump body, which acts as a stator, causes volumes of gas being pumped between an inlet and an outlet to be trapped between the threads of the rotors and the internal surface and thereby urged through the pump as the rotors rotate.
During use, heat is generated as a result of the compression of the gas by the rotors. Consequently, the temperature of the rotors rapidly rises, most notably at the stages of the rotors proximate the outlet from the pump. By comparison, the bulk of the stator is large and so the rate of heating of the stator is somewhat slower than that of the rotor. This produces a disparity in temperature between the rotors and the stator which, if allowed to build up unabated, could result in the rotors seizing within the stator as the clearance between the rotors and the stators is reduced.
It is known, for example from our International patent application no. WO 2004/036049, to provide a system for cooling the rotors of a screw pump in which a coolant is conveyed into, and subsequently out from, a cavity formed in the end of each rotor of a screw pump. Whilst able to provide effective cooling of the rotors, such a system tends to be relatively expensive to implement, both in view of the complexity of the system and the cost of the components of the system.